Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2000-06-19
2004-02-17
Gulakowski, Randy (Department: 1746)
Metal working
Method of mechanical manufacture
Electrical device making
C029S623500, C429S144000, C429S249000, C429S306000
Reexamination Certificate
active
06692543
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for manufacturing a lithium ion secondary battery comprising a separator containing electrolyte, a positive electrode and a negative electrode facing each other with the separator therebetween. More specifically, the present invention relates to a method for manufacturing a battery which can take optional form such as a flat type and is compatible with adherence and electric connection among a separator, a positive electrode and a negative electrode.
BACKGROUND ART
There is a great demand for miniaturization and weight reduction of portable electric appliances, and their realization requires improvements in battery performance. In recent years, various batteries have been developed and improved for improvements in the battery performance. The expected improvements in the battery performance are higher voltage, higher energy density, higher resistance to load, formability for optional form, safety and the like. A lithium ion secondary battery has been improved enthusiastically even in a recent year, since it has the highest voltage in a single battery in all kinds of batteries present and can realize higher energy density and higher resistance to loads.
A lithium ion secondary battery contains a positive electrode, a negative electrode, and an ion conducting layer sandwiched between these electrodes as a main component. In the practical lithium ion secondary battery, the positive electrode is generally made by mixing an active material powder such as a lithium-cobalt oxide composite with an electronic conductive powder and a resin to bond these powders, and applying the mixture onto an aluminum current collector to form it into a plate. And the negative electrode is generally made by mixing a carbon-based active material powder with a binding resin, and applying the mixture onto a copper current collector to form it into a plate. The ion conducting layer is generally made of a porous film such as polyethylene or polypropylene impregnated with a non-aqueous solution containing lithium ions.
For example,
FIG. 9
shows a cross sectional view of the structure of a conventional cylindrical lithium ion secondary battery disclosed in Japanese Unexamined Patent Publication No. 83608/1996.In
FIG. 9
, 1 is a solid casing made of stainless or the like which also serves as a negative electrode terminal, and
2
is an electrode assembly stored in the solid casing
1
. The electrode assembly
2
comprises a positive electrode
3
, a separator
4
, and a negative electrode
5
, which are coiled together. The electrode assembly
2
must give pressure from outside to its electric surface in order to maintain the electric connection among the positive electrode
3
, the separator
4
and the negative electrode
5
. To maintain all the contacts inside the surface, the electrode assembly
2
is stored in a strong metal casing. In the case of a square-shaped battery, strip-like electrode assemblies tied in a bundle are stored in a square-shaped casing and pressed with an external pressure.
As described above, in a commercially available lithium ion secondary battery, a strong solid casing made of a metal or the like is used as a means to adhere the positive electrode to the negative electrode. Without the solid casing, a distance between the electrode surfaces becomes far each other, failing to maintain the electric connection between the electrodes via the ion conducting layer (a separator impregnated with a non-aqueous electrolyte), thereby deteriorating the battery properties. Since a volume and a weight of the solid casing is large in the whole battery, it decreases energy density in a battery unit volume or a unit weight, and also limits the possible form of the battery due to the stiffness of the solid casing. Thus, it is difficult to obtain a desired form.
Under those circumstances, in order to realize a reduction in weight and thickness, a lithium ion secondary battery which does not need a strong solid casing have been developed. The key point to the development of the battery which does not require a strong solid casing is to successfully maintain the electric connection between the positive electrode, the negative electrode and the ion conducting layer sandwiched therebetween without applying an external pressure. One proposed method of joining the electrodes with the ion conducting layer without external force is to use a resin or the like.
For example, Japanese Unexamined Patent Publication No. 159802/1993 discloses a method for combining an ion conductive solid electrolyte layer, a positive electrode, and a negative electrode by heating using a thermoplastic resin binder. According to the method, these electrodes are joined with each other by uniting the positive electrode, the negative electrode and the solid electrolyte layer, so that the electric connection between these electrodes and the solid electrolyte is maintained without applying any external pressure, which makes it possible to function as a battery.
The conventional lithium ion secondary battery using a strong solid casing to secure adherence and electric connection between the positive and negative electrodes and the separator has a drawback that the solid casing, which is not included in the electric generator, makes up a large proportion of the entire battery in weight and volume, becoming a disadvantage to manufacture a high energy density battery. It seems possible to use an ion conducting adhesive resin to adhere a positive electrode, a negative electrode and the separator, but there arises a problem that when the positive and negative electrodes are simply adhered to the solid electrolyte (corresponding to a separator impregnated with an electrolyte) via an adhesive resin, too large resistance of the ion conductive adhesive resin layer causes the ion conductive resistance between the electrodes to increase, thereby deteriorating the battery properties.
In the example disclosed in Japanese Unexamined Patent Publication No. 159802/1993 wherein the positive and negative electrodes are joined with the solid electrolyte via a bonding agent, the interface among the positive and negative electrodes and the solid electrolyte is covered with the bonding agent, so that the battery is inferior to a battery with a liquid electrolyte in terms of conductivity and battery performance due to increased resistance between the electrodes. Even if a bonding agent having ion conductivity is used, it is still difficult to obtain the same level of battery performance as the liquid electrolyte.
The present invention, which has contrived as a result of hard study on the preferable method of adhering to the separator and the positive and negative electrodes to solve the above-mentioned problems, has an object of providing a method for manufacturing a rechargeable lithium ion secondary battery capable of adhering a positive and negative electrodes and a separator without a strong solid casing or without increasing resistance between the electrodes, and of having high energy density, being thinner, and being any desired form.
DISCLOSURE OF INVENTION
The first method for manufacturing a lithium ion secondary battery of the present invention comprises the following steps; a step of preparing a positive electrode obtained by joining a positive electrode active material layer with a positive electrode current collector, a negative electrode obtained by joining a negative electrode active material layer with a negative electrode current collector, and a separator arranged between the positive electrode and the negative electrode, a step of supplying a second solvent different from the first solvent to the applied adhesive resin solution after applying an adhesive resin solution, wherein an adhesive resin is dissolved in a first solvent, to the separator, and a step of forming an electrode laminate by laminating the positive electrode and the negative electrode to the separator.
By this method it is possible to join the active material layer with the separator without using a solid casing
Aihara Shigeru
Aragane Jun
Hamano Kouji
Inuzuka Takayuki
Murai Michio
Crepeau Jonathan
Gulakowski Randy
Mitsubishi Denki & Kabushiki Kaisha
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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